Homework with allocate subnet IP address

Question

I'm having difficulty solving a subnet allocation homework problem.
Assume that a university has an address block 128.205.224.0/19. It has to allocate addresses for 2 departments' networks, each of size 1800, and for 4 offices, of sizes 550, 600, 650, and 750 nodes respectively. Assuming that the university network allocates addresses sequentially from the beginning of the allocated allocated address space, what are the prefix allocations for these subnetworks?

I first thought in this way:
There should be 6 subnets in the network. So I need 3 bits for the subnets. So 3 + 19 = 22 bits should be the network bits. Then there are only 10 bits left. 2^10 = 1024 < 1800, so this cannot work.

Could you guys give me a hint or some thoughts for solving this problem?

Answer 1

One hint is to make sure you have a clear view of what the network masks cover, and charts like this make this easy: http://www.lumbercartel.ca/glossary/netmask.pl

Some additional detail is important too though -- is it one big network that shares in the use of an entire /19, or are the networks being split up to exercise the use of subnets or simplify routing at the different locations? You mentioned "allocating addresses" but that's not necessarily the same as subnetting.